C-nitrosodiarylamines, by which is meant diarylamines where one of the aryl groups bears a nitroso group, and which can be represented by the formula ArNHAr--NO, are important intermediates in the production of articles of commerce. For example, p-nitrosodiphenylamine (PNODPA), which currently may be the most useful representative of the class of C-nitrosodiarylamines, is generally the precursor of p-aminodiphenylamine, whose derivatives are widely used as antioxidants and antiozonants.
Generally, C-nitrosodiarylamines are prepared by acid catalyzed rearrangement of the isomeric N-nitrosodiarylamines, represented by the formula Ar.sub.2 N--NO. As the precursor of PNODPA, N-nitrosodiphenylamine (NNODPA) may be viewed as the archtypical representative of the class of N-nitrosodiphenylamines.
The difficulties inherent in the preparation of PNODPA by acid catalyzed rearrangement of NNODPA are diverse. Because both PNODPA and NNODPA are carcinogens, there is obvious risk attending handling. If too high an acid concentration is used in the rearrangement of NNODPA, denitrosation occurs to afford tars and other color bodies difficult to remove, even from subsequent products. It is also known that the presence of water in amounts greater than about 12% adversely affects rearrangement in the presence of acids. Additionally, both PNODPA and NNODPA are thermally labile. Thus, temperatures in excess of about 60.degree. C. commonly cause sufficient decomposition to afford undesirable color bodies and other unwanted materials.
The N-nitrosodiarylamines are usually prepared by nitrosation of diarylamines, represented by the formula Ar.sub.2 NH. Thus, the overall chemistry involved in preparing C-nitrosodiarylamines is described by reactions (1) and (2). ##EQU1## Because both reactions are usually performed in acidic media, in which both C- and N-nitrosodiarylamines form salts of the respective acid, it is to be explicitly understood that reference to C-nitrosodiarylamine or N-nitrosodiarylamine includes both the free base and a corresponding salt, unless noted otherwise.
Typically, the prior art methods of preparing C-nitrosodiarylamines utilize two discrete stages or sequences of operations. One stage corresponds to reaction (1), further characterized in that discrete operations are performed after formation of, but as a necessary prerequisite to rearrangement of, the N-nitrosodiarylamine. For example, the N-nitrosodiarylamine frequently is isolated prior to its rearrangement. As another example, the solution containing the N-nitrosodiarylamine may be treated so as to remove water, as by azeotropic distillation. Whatever the operation or treatment, the N-nitrosodiarylamine is thereafter rearranged to the C-nitrosodiarylamine in a second discrete stage whose operations correspond to reaction (2).
In contrast to the aforementioned staged method of preparing C-nitrosodiarylamines from diarylamines, the present invention relates to a process whereby the diarylamine is converted to product in but a single stage, as represented by reaction (3). ##EQU2## For the purposes of the present application, a single stage process is defined as one where two discrete chemical changes (corresponding to reaction (3)) are achieved by the continued addition of a reagent to a reaction mixture.
Single stage methods of preparing C-nitrosodiarylamines have been propounded. U.S. Pat. No. 2,046,356 describes a process of reacting a mixture comprising a solution of a diarylamine in an anhydrous alcohol containing suspended alkali metal nitrite with an anhydrous alcoholic solution of hydrogen chloride. Although the thrust of U.S. Pat. No. 2,782,235 is the rearrangement step, reaction (2) above, catalyzed by an aqueous solution of a hydrohalic acid, provided certain alcohols are present in the reaction medium, the patentees disclose also a single stage process whereby an aqueous solution of the hydrohalic acid acts on an alcoholic solution of a diarylamine containing suspended alkali metal nitrite. The patentee's claim that the amount of water present is not critical is contrary to the observations of the instant applicant.
In a series of related patents, Roberts describes a single stage process for the conversion of diarylamines to C-nitrosodiarylamines in alcoholic media containing an alkali metal nitrite by utilizing a chlorine containing compound to react with the water present to generate hydrogen chloride and maintain the reaction mixture in a substantially anhydrous condition. The chlorine containing compound is phosgene and other acid chlorides in U.S. Pat. No. 2,495,774; phosphorous trichloride and oxychloride in U.S. Pat. No. 2,560,892; sulfur dichloride and tetrachloride in U.S. Pat. No. 2,560,893; and silicon tetrafluoride in U.S. Pat. No. 2,560,894. The patents' stress on substantially anhydrous conditions is an important distinguishing feature.
The process of this application is a single stage method of preparing C-nitrosodiarylamines from their corresponding diarylamines. The success of the claimed invention rests on two separate and independent critical features. First, the described process requires the presence of water in a critical amount ranging from about 0.2 to about 18 gram per mole of diarylamine. Second, the success of the process also is dependent upon the addition of acid below the surface of the reaction mixture. The combination of these two features help define a novel single stage method of preparing C-nitrosodiarylamines from diarylamines in unusually high yield and with gratifyingly high purity.